This invention relates to an electric double layer capacitor used as a power supply for an automobile and a power supply for storing electric power, a control method thereof, an energy storage system using it, and improvement of a secondary cell.
The electric double layer capacitor is an electrochemical device consisting of a positive electrode, a negative electrode, a solution containing a cation and an anion (electrolyte). It can store and discharge electric charges by utilizing a very thin electrical insulating layer formed on interfaces between the positive and negative electrodes and the electrolyte, respectively, namely the electric double layers, as a dielectric, and adsorbing and desorbing the anion on the positive electrode and the cation on the negative electrode, respectively.
It is a widely held view that such an electric double layer capacitor does not entail chemical reactions and therefore has a longer life than chemical secondary cells because theoretically electric charges are stored and discharged by the positive electrode and the negative electrode adsorbing and desorbing the anion and the cation, respectively.
For example, the electric double layer capacitor that has a large energy and a long life is also disclosed by JP 2002-359155 A.
However, in the above-mentioned conventional technology, a very small amount of impurities existing in the electric double layer capacitor causes chemical reactions during discharge and charge, which causes a balance of electrode potentials of the positive electrode and the negative electrode to move to a noble side or base side. Moreover, in the presence of oxygen etc., an oxidation reaction without charge transfer advances and the electrode potential moves to a noble side. If these phenomena continue, the electrode potential reaches to a potential at which the electrode material and the electrolyte material are oxidized or deoxidized and degradation of these materials proceeds, which poses a problem of shortening a life of the electric double layer capacitor.